The principal investigator (P.I.) for this award application has previously been productive in evaluating potential research and clinical applications of myocardial contrast echocardiography in the setting of coronary artery disease. This past research experience has provided the P.I. with a basic understanding of ultrasound physics and perfusion imaging. The aim of this proposal is to determine whether contrast- enhanced ultrasound (CEU) may be used to noninvasively evaluate microvascular endothelial function and to detect early microvascular endothelial injury. CEU is performed by intravascular injection of microbubbles which produce backscatter of ultrasound during their microvascular transit through tissue. Microbubbles used to produce opacification during CEU are ideal flow tracers since they have a similar rheology as red blood cells and do not alter hemodynamics. Protocols have been developed for this proposal to determine whether endothelial function can be assessed in vivo by measuring changes in blood volume (BV) that occur secondary to microvascular vasodilation. A novel CEU technique has the potential to discriminate between arteriolar and capillary sites of BV changes. Studies will assess responses to insulin, which may cause both endothelial-dependent vasodilation and capillary recruitment, in the forearm flexor muscle groups in normal volunteers. These results will be correlated with microvascular responses to insulin in rat spinotrapezius muscle, visualized using intravital microscopy. Under conditions known to produce endothelial injury, albumin microbubbles appear to adhere to the vascular wall and, hence, persist in the microcirculation. A series of protocols have been developed in canine models to determine the mechanism of this interaction during cardioplegic arrest and in myocardium that has undergone reperfusion after coronary occlusion. Results will be correlated with direct observations of microbubble behavior and microvascular responses using intravital microscopy in the spinotrapezius muscle of the rat. The protocols listed above require a comprehensive understanding of microvascular perfusion, mathematical modeling, and ultrasound physics, for which Sanjiv Kaul, MD will serve as mentor. Klaus Ley, MD, PhD will guide microvascular studies and will advise in the field of endothelial function and injury. Under this guidance, the P.I. will attain the unique ability to bridge concepts in vascular biology and endothelial function with non-invasive imaging. Moreover, if the hypotheses of the research proposal are correct, CEU will offer invaluable insights into microvascular endothelial function, not currently available using any other technique and will provide the basis for the use of a noninvasive method to assess microvascular endothelial function in vivo.